There is always more control with manual transmission compared to automatic. Being able to use the gears to slow down is a massive bonus,
especially when descending hills. 1st gear low will keep the 4x4 descending slowly with little need for braking.
up hills in whatever desired gear is another bonus.
Manual gear boxes won't overheat.
The down side to manual transmission is really only general sand driving if the sand is soft, as loss of momentum can happen between gear changes.
After market options: other than completely changing the gear box there may be some options on different ratios but will depend on make and model.
Driving on soft sand with an auto is quite handy as the gear changes are smooth and no loss in momentum is suffered.
However driving down steep hills can be a problem, due the 4x4 wanting to constantly gain speed and change gear. This problem will cause the driver to continually apply the brakes possibly overheating the brakes.
This can cause a brake fade which pretty much renders the brakes useless, sending you to the bottom a bit too fast (should only really happen on a very long and steep hill or continuous steep hills one after another).
On hot days Autos can be prone to overheating.
After market options: automatic transmission coolers are available for most 4x4s and will take care of the overheating problems.
The transfer case is what makes 4x4 high and low range work. When changing the 4wd gear lever or button it makes the transfer case change gears much like a regular gear box.
High range 4 wheel drive is generally best used on gravel tracks or compacted surfaces which less resistance and faster traveling speeds are possible.
Low range 4 wheel drive is used when slower speeds are required. It enables more torque and power at lower speeds which assist heavily in moving through tough, boggy, soft sand terrain and also allows heaps more control climbing over rocks and logs.The highest speed in low range is about 45-60kph.
If you’re into rock crawling and constant hard-core off roading then a crawl box might be worth looking at.
Sometimes low range is just not low enough; most of us just keep using the clutch when taking on these tough sections that require lower speeds than standard low range.
If doing mostly rock crawling it does take its toll on the clutch. If just now and then there will be no reason to worry about the clutch.
A crawl box will add extra low gears, so low that even a petrol engine which revs/idles higher than diesel engines can master ultra-low speed crawling.